The subject invention relates to a method for determining the information capacity of a printed symbol communications channel. More particularly, it relates to measurement of the information capacity of a paper channel including a symbol input defining symbols to be printed, a bitmap generator responsive to said symbol input to generate input bitmaps representative of corresponding input symbols, a printer responsive to said input bitmaps to produce printed symbols substantially determined by said bitmaps on a substrate, and an imager to capture images of said printed symbols from said substrate and generate corresponding image signals.
As used herein the term “paper channel” refers to a communications channel, part of a complete communications channel, where information is input as a sequence of symbols; the sequence is coded, typically by a bitmap generator, as a corresponding arrangement of symbols, which are printed on a substrate; and, the substrate is scanned to generate an image signal (hereinafter sometimes “image”) as output of the paper channel. Typically, the image signal is then processed by a recognition system to determine the input symbol sequence. The input symbol sequence can also incorporate redundancies so that an error correction system can process the output of the character recognition system to recover more accurately the input symbol sequence.
The complete communication system, sometimes referred to herein as the printed symbol communications channel, from input to recovery of the symbol thus involves the paper channel and the recognition system and possibly an error correction system. The information capacity of the complete channel is limited by the capacity of the paper channel. However the limitations of the paper channel can be masked by the effects of the recognition system and error correction system. Thus, for example, when evaluating a bar code printer it can be difficult to separate the effects of the recognition and error correction systems from the print quality characteristics of the paper channel. Particularly, effects of changes in the coding of the bar code generator (i.e.graphic design of the symbols) can be difficult to identify. System developers can be misled by experiments performed with a paper channel and an imperfect recognition system. For example, a change in the bar code generator coding that reduces the paper channel information capacity can show improved overall reading. Thus, the problem of optimizing the whole system is subverted by accepting a change which improves the matching to a to suboptimum recognition system.
The term “bitmap” as used herein refers to the ideal or nominal symbol design which is stored in and sent to the printer from the “bitmap generator”. Actual storage of the “bitmap” can be in any convenient form such as an actual bitmap, line art, or simply a signal to print a particular symbol such as occurs with a line printer. Where an actual bitmap is stored, resolution of the stored bitmap and the scanned image need not be the same. Whatever form the actual storage of the symbol design takes, for purposes of the following analysis it is assumed, without loss of generality, to be transformed into a lattice of pixel values, i.e., an actual bitmap, having the same resolution as the scanned image.
Thus, it is an object of the subject invention to provide a method for evaluating the information capacity of a paper channel and the effects of design changes on that capacity.